FIG. 1 shows a wireless communication system. Referring to FIG. 1, a wireless communication system 100 includes a plurality of base stations and a plurality of terminals 120. The wireless communication system 100 may include a homogeneous network or a heterogeneous network. The heterogeneous network refers to a network in which different network entities such as a macro cell, a femto cell, a pico cell and a relay node coexist. A base station is generally a fixed station which communicates with a terminal and base stations 110a, 110b and 110c respectively provide services to specific geographical areas 102a, 102b and 102c. In order to improve system performance, the specific area may be divided into a plurality of sub areas 104a, 104b and 104c. The sub area may be referred to as a cell, a sector or a segment. In case of an Institute of Electrical and Electronics Engineers (IEEE) 802.16 system, a cell identity is allocated based on the entire system. In contrast, a sector or segment identity is allocated based on a specific area in which each base station provides a service and has a value of 0 to 2. Terminals 120 are generally distributed in a wireless communication system and may be fixed or mobile. Each terminal may communicate with one or more base stations at a certain time in uplink and downlink. The base station and the terminal may perform communication using Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Single Carrier-FDMA (SC-FDMA), Multi Carrier-FDMA (MC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA) or a combination thereof. In the present specification, the uplink refers to a communication link from a terminal to a base station and the downlink refers to a communication link from a base station to a terminal.